Helicopter rotor systems typically employ stops to limit the amount of vertical deflection experienced by the outer portions of the rotor blades relative to the flap hinge of the blade. These may include stops for limiting both downward, or “drooping,” deflection, and upward, or “flapping” deflection. Stops for limiting downward deflections are called “droop stops,” and stops for limiting upward deflection are called “flap stops.” Droop stops are required to prevent the rotor blade from contacting other objects, such as the ground, bystanders or other parts of the helicopter. Flap stops are required to prevent the rotor blade from “sailing” up uncontrollably, and thereby damaging rotor components.
Of importance, the amount of flapping or drooping that the blade is permitted to experience depends on whether the blade is rotating. Occasionally, larger flapping motions are required when the blade is rotating than when it is not. This is because, when the blade is rotating, it is exposed to a relatively large centrifugal force that acts on it radially, i.e., along it long axis. This force increases the stiffness of the blade, and additional flapping of the blade is then acceptable, and indeed, may be necessary, for proper blade operation. However, when the blade is not rotating, and the additional stiffening imparted to it by centrifugal force therefore absent, the blade tends both to sag more, due to the effect of gravity, and to sail more, e.g., as a result of wind gusts acting on it. Therefore, to prevent rotor damage, the flapping and drooping limits are more restrictive when the blade is either not rotating or rotating slowly.
It is known in the prior art to control the amount of flapping or drooping motions of the blade by the use of “stop mechanisms” that are actuated by centrifugal force (herein, “CF”). Thus, when the rotor is not turning, the mechanisms operate automatically to insert a stop, called an “interposer,” between portions of the blade and the rotor hub that restricts flapping or drooping of the blade about its flap hinge to a greater extent than when the rotor is turning. However, in addition to simply inserting an interposer to limit flap or droop for the rotor blade, it is important that the mechanism also be arranged such that the “re-insertion point,” i.e., the rpm, or rotational speed of the rotor, at which the interposer engages, be carefully “tuned” for the mechanism to function properly. Thus, if the re-insertion does not occur at the correct rpm, component damage and/or aircraft performance problems can result.
Additionally, any rotorcraft utilizing a blade-folding mechanism, e.g., for compact storage of the rotorcraft in a hangar or aboard a aircraft carrier, almost always requires a CF flap stop mechanism simply to enable the folding operation to be effected properly. Folding operations are highly susceptible to interference caused by wind gusts, and typically require that all flapping motion to be locked-out during the folding sequence. CF stop mechanisms are thus ideal for this application.
In the case of the dual-rotor CH-47 “Chinook” helicopter, the aft rotor system is tilted forward over the fuselage, thus making a blade-aircraft strike inevitable during rotor shut-down without tighter blade droop control. This tighter droop control requirement can be achieved using a prior art CF-actuated droop stop mechanism, and the CH-47 Chinook Helicopter is currently equipped with such a mechanism. The prior art CF droop stop mechanism utilizes an exposed linear-compression-spring-and-weight system. When the rotor begins to spin, a centrifugal force is applied to the weight of the mechanism, thereby causing it to be accelerated radially outward. This weight is connected by an arm to an interposer that limits droop of the rotor blade. Thus, when the weight moves radially outward, the interposer is moved out of the way, thereby exposing a stop surface that allows more droop of the blade to occur.
However, the prior art mechanism has a number of disadvantages associated with it, including its relatively large size and weight, complexity of installation and configuration for correct operation, and its susceptibility to damage due to the exposure of its components to the elements, e.g., icing conditions, thereby risking mechanical failure and damage to the aircraft caused by mechanical interference between the components of the mechanism.
Further, although the CH-47 Chinook helicopter currently utilizes only a conventional CF droop stop mechanism, it could, along with many other helicopter designs, benefit advantageously from the use of a CF flap stop mechanism. An appropriately designed CF flap stop mechanism could limit most flapping (i.e., upward motion) of the blades when the rotor is not turning, thus reducing the risk of aircraft damage due to wind gusts. However, the relative size and complexity of the prior art design now being used prohibits its use as a flap stop on the CH-47 Chinook and other helicopters.
Accordingly, there is a long felt but as yet unsatisfied need in the rotorcraft field for a CF-actuated helicopter blade flap- or droop-stop mechanism that is substantially smaller in size, lighter in weight, easier to install and configure for reliable operation, and whose susceptibility to damage and malfunction due to exposure to the elements is substantially less than that of current designs.